Dave-
For comparative perspective, look for a paper by myself, Robert Heimann
and Shadreck Chirikure that will appear in JAS when I can finish the
minor revisions requested by the reviewers. This is on the chemistry,
structure and technology of precolonial tin slags at Rooiberg, South
Africa. These are recent (ca. 1650-1850 AD) but chemically and
structurally they strongly resemble early tin slags (i.e. blowing house
and prehistoric) sites from Cornwall and Devon, like Crift Farm.
You should also look at a PhD dissertation by the geologist Dori
Farthing entitled "The Mineralogy of Tin Slags" (Johns Hopkins
University, Baltimore, 2002). Most of this is on prehistoric and
historic tin smelting in Cornwall and Devon. Dori has been a member of
this list - not sure if she still is.
The Cornwall and Rooiberg prehistoric slags are generally not fayalitic
before the introduction of the reverberatory furnace. The iron content
was generally kept low to avoid formation of excessive amounts of
hardhead (FeSn and FeSn2), so the slags tend to be glassy, with both tin
and silicon as glass-formers. Much of the Rooiberg slag derives from
hard-rock mining of cassiterite-tourmaline ores and consists almost
entirely of glass, with a small volume percentage of complex spinels
containing tin, but we also have slags that are obviously smelted from
placer deposits - the ores contained zircon and titanium minerals as
well as cassiterite. These slags contain much more crystalline matter -
relict zircons, and rutile, zirconolites and zirkelite crystallized from
the melt. Slags with high tin contents contain laths of reprecipitated
cassiterite, and slags of all types contain tin prills. Fayalite and
wustite are not usually present.
The published prehistoric and blowing house slags from SW England are
from placers that often contain a lot of titanium minerals. Dori
Farthing's dissertation is the only source of information, to my
knowledge on the mineral phase in these slags. She notes that the slags
are dominantly glassy, with minor relict quartz, feldspar, xenotime and
zircon, along with ilmenorutile, spinels, mullite and tin prills formed
from the melt. The reverberatory slags have hgher iron content and have
a much wider range of mineral phases.
I hope this helps.....
David Starley wrote:
> Dear all,
> Looking at some Roman metalworking debris recently I was reminded of
> some initial work on similar debris from the Trispen Bypass
> excavations (HMS News Spring 1997) where XRF seemed to show that that
> a site initially thought to be iron smelting had (also?) been involved
> in tin working/smelting. Hopefully I'll get a chance to XRF the new
> material to see whether this also contains traces of tin.
>
> Meanwhile I just wondered whether anyone has come across any other
> early tin smelting evidence in recent years to fill in some of the
> long-standing gaps in our knowledge of the extraction of this metal
> before the medieval period. The big question is whether we should be
> expecting glassy material, as found at Crift Farm (Medieval) or
> whether more debris of more fayalitic composition is being assumed to
> indicate iron working only.
>
> Any thoughts?
>
> Cheers,
> Dave
>
>
>
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